Concentrating and bleaching sulphuric acid



Patented Dec. 15, 1931 UNITED STATES PATENT OFFICE ARTHUR HOUGH, OFPASSAIG, NEW JERSEY GONCENT'RATING AND BLEAGHING SULPHURIC ACID 7 NoDrawing. Application filed January 11, 1930. Serial No. 420,284.

The present invention relates to concentration and bleaching ofsulphuric acid containing carbonizable organic matter, and

more particularly to sulphuric acid which has been used for theparchmentizing of paer or waterleaf.

Whereas, my invention can be applied to sulphuric acid containing manykinds of organic matter, which can, by the application of heat bedecomposed and carbonized, the invention will be described here as itrelates to the bleaching of sulphuric acid from parchmentizingprocesses.

In making vegetable parchment the waterleaf paper is passed through abath of cold sulphuric acid (for example sulphuric acid of about 68 to 73% strength, and preferably between 70% and 71% strength) attemperatures of about 50 to 60,F. The sheet being treated mayconveniently be a 23 pound paper or a 25 pound paper, and preferably apaper which is somewhat porous. The parchmentized paper is then washedwith cold water or wi h cold dilute acid and then with cold water, untilthe paper is substantially free from free acid, after which the paper isdried and subsequently rolled up or cut into sheets of the desired size.The dilute sulphuric acid, forming a by-product is, in some plants,reconcentrated for reuse in the parchmentizing process.

During the process of parchmentizino paper or waterleaf some of thesurface fibres of the paper are, by the action of the machinery andacid, removed from the surface of the paper into the sulphuric acid ofthe parchmentizing bath or into the washing bath, and these fibreshaving been parchmentized with the paper from which they are detached,constitute what is known in the art as fuzz or amyloid, and during theconcentration of the sulphuric acid, this amyloid or fuzz together withany organic matter dissolved by the acid, is wholly or partiallydecomposed, which blackens the acid, rendering it unsuitable forparchmentizing further amounts of paper, if the best results aredesired. This black color must therefore be removed.

Bleaching of the acid has heretofore been accomplished by one of severalreagents such as permanganate or bichromate or the like. The use of suchoxidizing agents however results in the formation of sulphates whichrapidly load up the acid to the point where considerable quantities ofthese sulphates are present in the acid, and these may deposit in thepipe lines and equipment employed in thefactory, causing trouble, delaysand ex- 10 pense.

Sulphuric acid from the parchmentizing machines may be reclaimed ataround 25% strength and if this is concentrated up to about 70% H SO atatmospheric pressure, then the temperature and time factors will be suchas to almost completely decompose the amyloid, and the acid may leavethe concentration apparatus at 130 to 150 0., and can be run into thebleaching tank without any further heating. If however, the weak acid beconcentrated to around 7 0% H 80 at reduced atmospheric pressure (i. e.under vacuum), then it is advisable to raise the temperature of the 70%acid to about 140 to 150 G., by any suitable means and hold it at thiselevated temperature, preferably while undergoing agitation, until theamyloid is wholly or sufficiently carbonized to respond well to thebleaching treatment.

In some instances sulphuric acid is reclaimedfrom the parchment machinesat a strength little below that required for parch mentizing, and by thesimple addition of a high strength acid such as oleum of selectedstrength, to compensate for the amount of acid lost in process, thesulphuric acid will be strong enough to employ again for parchmentizingthe paper. It will however still contain amyloid and this must becarbonized and bleached out, otherwise the acid would eventually becomeloaded with the amyloid and useless. To prepare such an acid forbleaching it is only necessary to hold it in a suitable vesselcontaining heating units, or heated from the out-side by means of asteam or hot oil jacket, or any suitable method, and preferably agitatethe acid in order to secure the carbonized amyloidin an extremely fine,almost colloidal condition, which condition is admirable for subsequentbe forced by means of a 5 tion of the black bleaching by the methoddescribed below. In this way, after a time it will be found that theamyloid will have decomposed and the acid will be quite black.

In accordance with the present invention employ ammonium nitrate eitherin the crystallized state or preferably in solution in water. If thecrystals are used, these must be put into the black acid in calculatedquantities, and to get the best results, brisk agitation of the acid bymechanical or other means should be continued until the desired degreeof bleaching is effected.

If a solution of ammonium nitrate is used (and this is the preferred wayto use it) the warm concentrated solution may be run into. the blackacid, preferably mixing with it, in the stream running from theconcentrator, or in the case of no concentration of the acid beingrequired, from the apparatus employed for carbonizing the amyloid. Thissolution, (which may for example be 70 to 80% strength) may be fed (say,at to 80 C.) from an elevated tank or it may suitable pump well downinto the black acid which effects good mixlng- I preferably provide anacid-proof box having considerable depth, and the hot acid from theconcentrator runs into the lower part of this box through a glass tube.I also provide a separate glass tube for delivering ammonium nitratesolution into close proximity to the stream of acid running from thConcentrator into the said box. The box may be kept at about 145 C. orat any event should be kept sufficiently hot to cause the sulphuric acidto immediately react with the ammonium nitrate, and give ofl nitric acidwhich in the nascent state at once attacks and destroys the carbonizedorganic matter in the; acid. V

The flow of ammonium nitrate solution will of course be regulated tosuit the condiacid, i. e. the amount of organic matter therein containedto be oxidized, or bleached out. On addition of the ammonium nitratewhether in the form of crystals or solution, the reaction is as follows:

This react-ion is fairly rapid, but the actual bleac-ing effect is notso rapid, depending largely on the temperature. It is thereforenecessary to maintain the acid at as high a temperature asis convenient,which of course may be done by inserting a heating coil, whichrmay be oflead, in the acid tank and employing steam or heated oil or any suitableheating medium for this purpose. It

is not advisable to use too much heat for this purpose, as some of theavailable nitric acid (resulting from the reaction of theammoniumnitrate with the sulphuric acid may be drivenoff reducing theefliciency of theprocess). The best temperature for this system ofbleaching is around 140 C. for acid at about H he time required undersuch conditions for a good end point in color, which is a light ambertint, is about 20 minutes, so if a continuous process of bleaching isdesired the capacity of the black acid tank should be regulated to thisend. The acid having been bleached, it is now necessary to remove theexcess of nitric acid, and the ammonium sulphate shown in the aboveequation. This removal is brought about automatically and without anyadditions of chemicals of any nature, only if the reaction is desired tobe speeded up it is best to raise the temperature of the bleached acidas high as possible, which of course can only be as high as the boilingpoint of the acid, which if 70% is around 170 C. and it is advisable tokeep the acid agitated during this process of denitration. Agitation bybubbling a regulated air current through the acid is good. A convenientway to do this is to have a second tank into which the bleached acidflows, this second tank having a separate heating coil employing anysuitable heating agent capable of keeping the bleached acid at thedesired high temperature. Under these conditions another reactionproceeds, which may be expressed as follows:

N203 2SO4 3H2O +H SO4 The entire series of reactions occurring may besomewhat complicated, but the above equation expresses the resultbriefly. Experience has shown that the process results in very goodbleaching, and the final outcome is bleached acid without a trace ofnitric acid or low oxides of nitrogen or ammonium sulphate present, allaccomplished by the addition to the black acid of one reagent only, andthat is ammonium nitrate. This reagent moreover does not add salts inthe form of sulphates to the bleached acid, for the reasons abovedescribed.

The quantity of ammonium nitrate solution employed will depend upon thedegree of blackness, of the acid, and this. in turn will depend upon agood many factors in the parchmentizing and washing processes. In afactory where 100,000 pounds of 70% bleached acid are made from wastedilute acid (about 20% strength)v each day, the

amount of ammonium nitrate may be from pounds to. 500. pounds, theblackness of the acid. The pump for introducing the ammonium nitratesolution is preferably adjustable so that it can be run faster pump canbe made greater or less, in order to deliver the best amount of ammoniumnitrate. solution into the sulphuric acid to be bleached.

The removal of the free nitric depending upon acid and or slower, or sothat the stroke of the low oxides of nitrogen is absolutely essentialwhen the bleached sulphuric acid is to be again employed forparchmentizing paper or waterleaf. If they are not removed, the productreferred to, viz., parchment paper will be of inferior quality due tothe action on the paper (cellulose) by nitric acid.

I have above referred to concentrating the acid to about strength beforebleaching. In some cases it may be more advisable to concentrate to asomewhat lower degree than this, and then eifcct bleaching by theprocess above described, followed if desired, by furher concentration.In most cases however I prefer to carry the concentration to a littleabove the concentration which is to be used in parchmentizing, since thesulphuric acid may become diluted a little during the bleachingoperation, by the water constituting the solvent of the ammonium nitrateand the water produced in the decomposition of the ammonium sulphate.

It will be understood that the reaction between sulphuric acid andammonium nitrate to form nitric acid occurs at all temperatures, but thesecondary reaction between the ammonium sulphate and nitric and nitrousacids requires relatively high temperatures.

I have referred above to adding the solution of ammonium nitrate whilewarm. The solution can be added cold, if desired. Ammonium nitratesolution when warm, will hold more of the nitrate, so that less waterneed be added if the solution is used warm.

I have referred to introducing this through a glass tube. A porcelain orearthenware pipe or tube can be used, but glass is transparent.

Having described my invention, I claim:

1. A process of concentrating and bleaching sulphuric acid containingorganic matter,

which comprises concentrating the acid and adding ammonium nitrate, andholding the acid hot until the acid is bleached, substantially asdescribed.

2. A process of concentrating and bleaching sulphuric acid containingcarbonized organic matter resulting from the carbonization ofdegradation products of cellulose, which comprises concentrating theacid by heating to at least 70% strength, and adding to the hot acid asolution of ammonium nitrate, substantially as described.

3. A process of concentrating, bleaching and denitrating sulphuric acidcontaining organic matter, which comprises concentrating the acid byheating to not substantially below 70%, adding ammonium nitrate, andholding the bleached acid hot long enough to bring about the reactionsbetween nitric acid and nitrogen oxides and' ammonium sulphate,resulting in a denitrated bleached acid, substantially as described.

4. A process of concentrating, bleaching and denitrating sulphuric acidcontaining organic matter, ing the acid to a which comprisesconcentratstrength sufficient for use in parchmentizing, adding ammoniumnitrate in solution below the surface of said acid, holding the acid hotuntil bleaching is brought about, agitating the acid during the additionof ammonium nitrate solution, then raising the temperature of thebleached acid to bring about more rapidly the secondary reactionresulting in denitration of the sulphuric acid substantially asdescribed.

5. A process of bleaching sulphuric acid containing organic matter,which comprises heating the sulphuric acid long enough to at leastpartially decompose and carbonize such organic matter to put it incondition to respond to the bleaching process, then addin ammoniumnitrate, keeping the acid su ciently hot for sufficient time tosufiicient- 1y bleach the acid, and to denitrate the same substantiallyas described.

6. A process of concentrating and bleaching the dilute sulphuric acidremaining from parchmentizing and similar operations containingdegradation products of cellulose, which comprises concentrating theacid to about 70% H 80 whereby a considerably blackened acid isproduced, adding to said acid, while hot, a solution of ammonium nitrateand heating the acid to aid in decomposing the ammonium nitrate andblowing with air to remove the last of the nitric acid liberated.

7. In concentrating and bleaching dilute sulphuric acid fromparchmentizing and similar operations, containing degradation productsof cellulose, the herein described steps of adding a solution ofammonium nitrate to the hot concentrated dark colored sulphuric acid,and supplying heat to the am 8. In the process of claim 7, introducinthe dark colored concentrated acid into a bat% of such acid, while thelatter is hot, and introducing a solution of ammonium nitrate, such twointroductions being effected in close proximity to each other, wherebythey mix promptly.

9. Process as in claim 7, in which the concentrated acid and the nitratesolution are both introduced at near the bottom of the bath of acid.

10. A process of bleaching sulphuric acid containing organic matter,which comprises heating the acid long enough to ensure decomposition ofthe organic matter, adding ammonium nitrate, holding the acid at atemperature and for a length of time necessary to sufiiciently bleachthe acid, then raising the temperature of the acid to accelerate thesecondary reaction to produce denitration, substantially as described.

11. A process of recovering sulphuric acid of a concentration adaptedfor parchmentizing which comprises heating the reaction product ofammonium nitrate and sulphuri'c acid containing ca-rbonization productsof cellulose, at about 0., long enough to bleach out the organic matterand raisin the temperature of the mixture to C., so as to effect thedecomposition of the ammonium sulphate.

In testimony whereof I affix my signature.

ARTHUR HOUGH.

